The present invention relates to a diagnostic device and a diagnostic method of an evaporated fuel processing system, in particular, to a diagnosis of a leak in the evaporated fuel processing system including a fuel tank.
The present application claims priority from Japanese Patent Application No. 2003-304914, the disclosure of which is incorporated herein by reference.
In order to prevent the fuel evaporated in the fuel tank from being released to the atmosphere, an internal combustion engine including the evaporated fuel processing system is known. In this system, the evaporated fuel (evaporated gas) generated in the fuel tank is temporarily adsorbed by an adsorbent disposed in a canister. Then, the adsorbed evaporated fuel is released to an air intake system of the internal combustion engine through a purge passage under predetermined operating conditions. However, if a component of the system is broken or exploded for some reason, the evaporated fuel is released to the atmosphere. In order to prevent such a situation from taking place, a leak diagnosis for determining whether there is a leak in the evaporated fuel processing system or not is executed (for example, see Japanese Patent Application Laid-Open Nos. 2001-41116 and 2003-56417).
In the leak diagnosis, using an intake negative pressure, the negative pressure is first introduced in the evaporated fuel processing system. After changing the negative pressure into a target pressure value (a predetermined value of the negative pressure), the evaporated fuel processing system is closed. Alternatively, using a pump or the like, a positive pressure is introduced in the evaporated fuel processing system, and then after changing the positive pressure into a target pressure value (the predetermined value of the positive pressure), the evaporated fuel processing system is closed. Then, the amount of the evaporated fuel leak from the evaporated fuel processing system is estimated on the basis of a change amount (difference amount) between an internal pressure value of the evaporated fuel processing system at a timing of the closing and that at the timing after elapse of a predetermined time from the closing timing, and thus it is determined whether the leak exists or not.
A pressure state in the evaporated fuel processing system, however, does not stabilize yet immediately after the closing, and also it takes some time to stabilize the pressure state. More specifically, an overshoot occurs which is a phenomenon that the internal pressure in the evaporated fuel processing system continues to decrease more than the target pressure value with an elapse of time just after closing in the case of introducing the negative pressure, or that the internal pressure in the evaporated fuel processing system continues to increase more than the target pressure value with the elapse of time just after closing in the case of introducing the positive pressure.
Then, in the conventional leak diagnosis, an internal state of the evaporated fuel processing system is hold only in a constant period after the closing in view of the occurrence of the overshoot in advance, and namely after the internal pressure of the evaporated fuel processing system returns to the target pressure value, it starts to calculate the change amount of the internal pressure. In the conventional method, however, the calculation of the change amount can not be processed until the internal pressure returns to the target pressure value. As a result, there occurs an inconvenience that it takes too much time for diagnosing the leak.